Doors are significant in preventing the spread of fire and as a barrier to noise. Many doors are equipped with locking mechanisms to allow entrance to certain people and keep out others.

​

When open, doors admit people, animals, ventilation or light. The door is used to control the physical atmosphere within a space by enclosing the air drafts, so that interiors may be more effectively heated or cooled. When closed, a door normally impedes the transfer of air from one side to the other. Similar structures that do allow air to be transferred through some form of a grillwork are called gates.

​

Doors may have an aesthetic purpose in creating an impression of what lies beyond; for example, keeping administrative and factory areas of a building separate. In less formal settings, doors may also be seen as a sign of the desire for privacy. As a form of courtesy and civility, people often knock before opening a door and entering a room.

​

Doors are often symbolically endowed with ritualistic purposes. For example, being granted access to a door, including the guarding or receiving of the key to that door, may have special significance. Similarly, doors and doorways frequently appear in literature and the arts in metaphorical or allegorical context, often as a portent of change.

​

In most cases the interior side of a door matches its exterior side but, in some other cases, there are sharp contrasts between the two sides, such as in the case of a vehicle door.

​

The earliest in records are those represented in the paintings of the Egyptian tombs, in which they are shown as single or double doors, each in a single piece of wood. Doors were once believed to be the literal doorway to the afterlife, and some doors leading to important places included designs of the afterlife. In Egypt, where the climate is intensely dry, there would be no fear of their warping, but in other countries it would be necessary to frame them, which according to Vitruvius (iv. 6.) was done with stiles (sea/si) and rails (see: Frame and panel): the spaces enclosed being filled with panels (tympana) let into grooves made in the stiles and rails. The stiles were the vertical boards, one of which, tenoned or hinged, is known as the hanging stile, the other as the middle or meeting stile. The horizontal cross pieces are the top rail, bottom rail, and middle or intermediate rails. The most ancient doors were made timber, such as those referred to in the Biblical depiction of King Solomon's temple being in olive wood (I Kings vi. 31-35), which were carved and overlaid with gold. The doors dwelt upon in Homer would appear to have been cased in silver or brass. Besides olive wood, elm, cedar, oak and cypress were used. A 5,000-year-old door has been found by archaeologists in Switzerland.